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J. of Supercritical Fluids 47 (2009) 598–610

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com/ locate /supf lu

ultiple unit processing using sub- and supercritical fluids

erry W. King ∗, Keerthi Srinivasepartment of Chemical Engineering, 3202 Bell Engineering Center, University of Arkansas, Fayetteville, AR 72701, USA

r t i c l e i n f o

rticle history:eceived 31 July 2008eceived in revised form 14 August 2008ccepted 14 August 2008

eywords:arbon dioxideritical fluidsultiple fluidsnit processesater

a b s t r a c t

The past twenty years has seen an expansion of the “critical fluid” technology platform with respect tousing or combining multiple types of unit operations and compressed fluids in both their sub- and super-critical states. Although supercritical fluid extraction (SFE) utilizing supercritical carbon dioxide (SC-CO2)has seen considerable industrial use, the high capitalization cost of building and running such produc-tion plants promotes expanding their use and functionality with respect to multi-unit operations andmultiple fluids. An evaluation of the current status of implementing such an approach is provided, and aperspective of future needs suggested based on the senior author’s 40 years of experience in critical fluidprocessing of agriculturally-derived materials and natural products. The use of hot compressed wateris no longer limited to just supercritical water processing, but can be advantageously exploited over awide range of temperatures and appropriate pressures to include extraction and reaction of targeted sub-strates, including bioactive natural products, biomass conversion for renewable fuels, and synthesis ofchemicals. As noted previously, combining SFE with fractionation methods using critical fluids, and/orreaction chemistry in critical fluid media can produce a variety of extracts or products [J.W. King, Sub-and supercritical fluid processing of agrimaterials: extraction, fractionation and reaction modes, in: E.Kiran, P.G. Debenedetti, C.J. Peters (Eds.), Supercritical Fluids: Fundamentals and Applications, KluwerPublishers, Dordrecht, The Netherlands, 2000, pp. 451–488.] however the modification of processing

equipment such as expellers, extruders, and particle production devices extend the use of the criti-cal fluid technology platform. Sequential unit processing using multiple fluids also has been reportedalthough demonstrated to a limited degree—however combining both compressed CO2 and water in aunit process can also impart some unique processing possibilities. The concept of multiple unit and fluidprocessing supports an environmentally benign or “green” production platform and is consistent withprocessing sustainable materials. Integration of critical fluids into existing processing concepts such as

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. Introduction

Supercritical fluids and their liquefied analogues have beenraditionally used in single unit operations, i.e. extraction, fraction-tion, using neat SC-CO2 or with appropriate modifiers [2]. Sincehe 1980s, almost 38% of the supercritical fluid extraction pro-esses have been devoted to extraction of food and natural products3]. Beginning in the mid-1980s, columnar and chromatographicechniques followed by reactions in supercritical fluids were devel-ped to facilitate supercritical fluid derived extracts or products

4], thereby extending the application of a critical fluids processinglatform beyond SFE. These newer developments were investigated

n part due to the complexity of many natural product matricesnd the desire to concentrate specific target components for food

∗ Corresponding author. Tel.: +1 479 575 5979; fax: +1 479 575 7926.E-mail address: jwking1@uark.edu (J.W. King).

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896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2008.08.010

ies, and synthesis based on methyl ester intermediates are discussed.© 2008 Elsevier B.V. All rights reserved.

nd other industrial uses, as illustrated in Fig. 1. This approach haseen summarized by the senior author and others in previous pub-

ications [1,5,6]. The perspective we offer here is somewhat biasedowards past and current studies which focus on advancing multi-le unit and/or fluid processing of agriculturally-derived materialsr natural products.

Despite this focus, the perspective we offer opens other optionsor applying critical fluids in general by embracing the idea ofandem processing using different compressed fluids. Commen-urate with the goal of environmentally benign processing is these of liquefied or supercritical CO2 for non-polar to moderatelyolar solutes, and compressed water between its boiling and criti-al points for more polar solutes and reactants. Ethanol, a naturally

erivable sustainable solvent, is suggested as a preferred co-solvento be paired with compressed water or carbon dioxide. This sim-le, renewable compressed fluids platform has many advantagesnd can be used to achieve multiple results, particularly when usedequentially, or in tandem with multiple unit processes.

J.W. King, K. Srinivas / J. of Supercritical Fluids 47 (2009) 598–610 599

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ig. 1. Integration of multiple unit critical fluid processing for processing naturalroducts.

We have found that the generic solvation properties of thewo principal critical fluids, CO2 and water, to be explained by anxtended solubility parameter approach [7,8]. Hence by adjustmentf pressure and temperature for CO2, or temperature in the casef water, one can optimize the solubility of solutes or reactants inhese media, or predict their miscibility, by comparing their relativeolubility parameters as a function of temperature and pressure.uch an approach has a practical value considering the molecularomplexity of many solute types processed in critical fluids. Theirolubility parameters or solute–fluid interactions can be explainedy using the Hansen three-dimensional solubility concept whichllows the application of functional group contribution methodsor calculating requisite physical property data as well as solute orolvent solubility parameters as recently reported by Srinivas et al.9]. As indicated by Fig. 2, the reduction in water’s total solubilityarameter with increasing temperature is largely due to a reduction

n the hydrogen-bonding propensity as reflected by its hydrogen-onding solubility parameter component, ıH [10]. Water does notttain the solvation properties of solvents like ethanol or methanolntil quite elevated temperatures which is in contrast to the oftenited dielectric constant concept which is invoked to explain theolvent properties of subcritical water [11].

The above solvent properties of water as described by theolubility parameter concept has some implications with regardo its use as a “green” solvent and as a substitute for ethanol

n hydroethanolic-based extractions which are GRAS (Generallyegarded as Safe)-approved food processing solvents. Its substi-ution for ethanol as a processing medium is highly desired to saven processing costs, its separation from water in solvent recycle

ig. 2. Three-dimensional solubility parameters of water as a function of tempera-ure.

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Fig. 3. Coupled processing options for critical fluids.

chemes, and oversight by revenue authorities. These are somef the factors which accelerate research in the use of subcriticalater for the extraction of natural products and nutraceutical food

omponents. This aids in considering the widespread utilization ofater and carbon dioxide as a part of the overall critical technologylatform [1].

Traditional oleochemical processing operations such as fatplitting or hydrogenation are often conducted under either sub-ritical or supercritical processes. Fat-splitting processes such ashe Twitchell process [12] or Colgate-Emery synthesis [13] uti-ize temperatures and pressures in excess of the boiling point ofater under the appropriate pressure, but below the critical pointf water to facilitate the hydrolysis of triglycerides to fatty acids.owever, it should be noted that these processes were frequently

nterpreted as steam-based hydrolysis rather than hydrolysis usingubcritical water. Hydrogenations using binary mixtures of CO2-H2r propane-H2 are supercritical with respect to the pure componentritical constants, but reaction conditions are conducted under lessense conditions due to the high temperatures involved, and hencehe cited rapid kinetics associated with hydrogenations conductednder these conditions [14] refers more to the accelerated massransfer effects as opposed to reactant solubility enhancement.nzymatic-based synthesis under supercritical conditions has beenargely confined to laboratory-scale studies [15–17] and the sensi-ivity of enzymes to operational parameters and their attendantigh cost suggests that their application maybe limited to the syn-hesis of high cost products.

The advantages of coupling processing options using criticaluids are illustrated in Fig. 3. Hence by combining different unitrocesses and sequencing them with the use of multiple fluidseld at operational densities by the application of different tem-eratures and pressures, one can obtain multiple products and

ptimize the extraction or reaction process. Several specific optionsre illustrated for the case of processing essential oils as noted pre-iously in Table 1 [18] using pressurized fluids. Here six discretenit processes are listed which include standard SFE with SC-CO2,

able 1oupled processing options for essential oils processing using pressurized fluids

rocess(1) SFE (SC-CO2)(2) SFF (SC-CO2)—Pressure reduction(3) SFF (SC-CO2)—Column deterpenation(4) SFC (SC-CO2/co-solvent)(5) SFF—(subcritical H2O deterpenation(6) SFM—(aqueous extract/SC-CO2)

rocessing combinations(1) + (2)(1) + (2) + (3)(1) + (4)(1) + (5)(1) + (5) + (6)

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tcbdabbsatscoVnabhattboicmiscible in the subcritical water medium. We have also used thisapproach for other biopolymers treated in subcritical water such ashemicellulose and chitin and rationalized the difficulty in dissolv-ing lignin-type polymers in subcritical water [7]. A similar approachalso has utility in understanding the subcritical water extraction

00 J.W. King, K. Srinivas / J. of Sup

FF (supercritical fluid fractionation) employing stage-wise pres-ure reduction, SFF as practiced using column-based deterpenation19], supercritical fluid chromatography (SFC), another variant ofFF called subcritical water deterpenation [20], and utilization ofSC-CO2 or LCO2 with a perm selective membrane described by

owsley et al. [21]. As shown in Table 1, combinations of these pro-esses can allow a total processing scheme to be conducted usingritical fluids. For example, a combination of processes (1) and (2)n Table 1 could be combined to yield a more specific compositionn the final extract. Unit process (1) if conducted by sequentiallyncreasing the extraction density when coupled with a sequencef let down pressures (unit process 2) can amplify the SFF effect.ikewise, by combining unit process (1) using SC-CO2 followed bypplication of unit process (2) utilizing subcritical H2O to deterpe-ate the extract from unit process (1), will yield a more specificnal product from the starting citrus oil. To obtain a more enrichednd/or concentrated product from the latter process, one could addnit process (6), a supercritical fluid membrane-based separationf the aqueous extract/ractions from unit process (5) as indicatedn Table 1.

We shall discuss several other combinations of unit processesnd fluids below based on both our research and those of othersited in the literature. Many of these examples are focused on therocessing of complex natural product mixtures, lipids and oleohemicals, and more recently the processing of biomass for value-dded extractives and conversion to fuel substrates, since we areost familiar in applying critical fluids in those areas.

. Multiple critical fluid processing platforms

In the broadest sense, multiple critical fluid processing involveshe integration of two or more fluids held under pressure applieds either mixtures or in a sequential manner for one or more unitrocesses. Listed in Table 2 are the most prevalent combinationshat have been utilized or have potential application in processevelopment. Solubility of solutes and reactants in SC-CO2 has beenxtensively studied and a recent tome has assembled much of thevailable data [22]. Likewise there is a fair understanding as to thehoice of a suitable co-solvent to pair with SC-CO2 to enhance theolubility of more polar solutes in the compressed CO2 medium,lthough binary phase equilibria data is not always available overhe desired range of pressure–temperature for such systems. This isritical if one is concerned with operating in the one phase super-ritical fluid region with respect to both components, however asoted by several investigators [23–25], there are several exampleshere processing can be done with SC-CO2–co-solvent systems in

he two phase region. This situation becomes of interest partic-larly when large amounts of an organic co-solvent are used inonjunction with SC-CO2 to enhance the solubilization of a solutehich exhibits limited solubility in neat SC-CO2. The critical ques-

ion then becomes whether another compressed fluid might better

e integrated into the design of the process.

As remarked previously, compressed water at high temperaturesnd pressures, i.e., supercritical water has been extensively inves-igated for many years. In the 1990s’ a similar but somewhat more

able 2ultiple critical fluid processing options

ode Example

ingle fluid Carbon dioxide, wateringle fluid + co-solvent Carbon dioxide + ethanolinary gases above their Tc Carbon dioxide + hydrogeningle fluid + dissolved gas Water + carbon dioxideequential fluids Carbon dioxide, then water

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cal Fluids 47 (2009) 598–610

iffuse focus on using water in its subcritical state with respect tots. Tc received attention due to its application as a reaction mediumo transform organic chemicals and biomass into targeted products26,27]. Concurrently in the field of analytical chemistry, subcriti-al water and other subcritical fluids were explored as alternativextraction solvents under external compression above their boil-ng points [28–30]. Analytical methods developed with the use ofressurized solvents essentially use subcritical fluids above theiroiling point—the pressure applied frequently is far in excess ofhat is required by inspection of the V–L curves for these fluids [31].nfortunately researchers in these disparate areas using water as aommon compressed fluid have not always recognized its generictility as a universal compressed fluid medium as well as “green”omplimentary solvent to compressed CO2 [7].

Recently we have attempted to explain the solvent charac-eristics of water using an extension of the solubility parameteroncept, i.e., a three-dimensional solubility parameter approach,y applying it to pressurized water [8,9]. Using the Hansen three-imensional solubility parameter approach coupled with SPHEREnd Hsp3D [32] software programs, we have studied the interactionetween subcritical water and complex organic solutes, includingiopolymers, as a function of temperature. Water under compres-ion has the ability by adjustment of the applied temperaturend pressure to serve as an extraction solvent as well as a reac-ion medium depending on what is desired. Residence time of theolute (reactant) in the aqueous medium thus becomes a criti-al parameter in conducting extractions above the boiling pointf water and for optimizing reaction conditions “higher up” the–L curve for water. There appears in our opinion the lack of ratio-ale design for choosing reaction conditions in subcritical water,lthough the semi-empirical “severity” parameter often-cited iniomass conversion studies is one attempt to quantify the requiredydrolytic conditions [33]. Using a simple solubility parameterpproach, as depicted in Fig. 4, in which the solubility parame-er for water as a function of temperature is plotted along withhe solubility parameters for cellulose oligomers (n = 1–10), it cane seen that the intercept between the solvent and the celluloseligomers corresponds to the chosen conditions for depolymeriz-ng the carbohydrate polymers [7]. This confirms that conditions areommensurate with those in which the biopolymer is dissolved or

ig. 4. Solubility parameter variation for subcritical water at different reduced pres-ures pr and cellulose oligomers (n = 1–10) with temperature.

J.W. King, K. Srinivas / J. of Supercritical Fluids 47 (2009) 598–610 601

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ig. 5. Variation of Hansen three-dimensional solubility parameter sphere of betulinith subcritical water and ethanol at different temperatures.

f target solutes from natural product matrices, such as silymarinsrom milk thistle, B-vitamins from brewers yeast, and anthocyaninsrom grape pomace.

The Hansen three-dimensional sphere method noted abovean also be used in choosing a subcritical fluid as an extractionolvent and optimizing the extraction conditions. The three-imensional solubility parameters of water and ethanol at aeference temperature (25 ◦C) are obtained from Hansen [32] andheir temperature-dependence calculated using the approach of

illiams [34]. The Hansen spheres were then generated using the

sp3D program in a Matlab platform. The inverted white trian-les which appear in Figs. 5 and 6 below indicate that the targetedolute is within the solubility sphere thereby ensuring high misci-ility with the subcritical fluid, while the dark triangles indicate

mmiscibility between the solute and the subcritical fluid refer-

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ig. 6. Variation of Hansen three-dimensional solubility parameter sphere ofyanidin-3-O-glucoside with subcritical water and ethanol at different tempera-ures.

ing to the compounds outside the Hansen solubility sphere. Thenverted white triangle on the other side of the center of the massf the sphere is the solute itself.

One study we have contributed to involves the extraction ofetulin from birch bark extracts using subcritical water or ethanols solvents. The experiments were conducted using subcriticalater maintained at 40–180 ◦C and 5 MPa using an analytical-based

xtraction system. It was found that a limited amount of betulinas extracted into subcritical water even at temperatures as high as

80 ◦C. However, similar studies conducted with subcritical ethanolielded high amounts of betulin at 90 ◦C and 5 MPa [35]. The three-

imensional solubility parameter Hansen sphere plots for betulin inater and ethanol respectively are shown in Fig. 5. Here the radiusf the Hansen sphere for the betulin–ethanol system is far less thanhat observed for the betulin–water system. This is indicative of

602 J.W. King, K. Srinivas / J. of Supercriti

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ig. 7. Variation in the Hansen sphere radius (MPa1/2) versus %ethanol in com-ressed water–ethanol extraction solvent.

he fact that subcritical ethanol is a far better solvent under thetated conditions than compressed hot water. A further study of theelative energy difference (RED) values for the two solute–solventystems indicates miscibility–solubility between 50 and 125 ◦C inhe betulin–ethanol system with a maximum betulin solubilityccurring at 100 ◦C [9].

Similarly in Fig. 6 are the Hansen spheres for the antho-yanin, malvidin-3-O-glucoside, in water and ethanol, respectively.he sphere plots indicate a greater miscibility of malvidin-3--glucoside in ethanol since the Hansen sphere has a lower

adius (7.23 MPa1/2) in the temperature range of 25–75 ◦C. For theater–malvidin-3-O-glucoside system, the corresponding solubil-

ty sphere occurs over a different temperature range and a REDadius of 10.35 MPa1/2, higher than that for ethanol [8]. While bothubcritical solvents dissolve the target anthocyanin at different con-itions, clearly ethanol would be the preferred subcritical solventith respect to anthocyanin miscibility and solubility.

The three-dimensional solubility parameter sphere approachan also be used to explain experimental results we have obtainedor the solubility and extraction of malvidin-3-O-glucoside inompressed-hydroethanolic solvent medium. The Hansen solubil-ty spheres were plotted for water–ethanol mixtures and theirorresponding interaction radii versus the composition of subcrit-cal water–ethanol mixture is shown in Fig. 7. The plot showsn initial substantial decrease in the interaction radius betweenhe anthocyanin and hydroethanolic mixture upon addition of0% ethanol to water (this is due to the substantial decrease inhe hydrogen bonding solubility parameter with the addition ofthanol to water). This is consistent with the enhanced extractionecorded for the flavonoid upon addition of ethanol to the extrac-ion medium. Note that although in Fig. 7 the minimum interactionadius occurs at 80% ethanol concentration, there is not a signif-cant difference with that recorded at 10% ethanol content. Thisas important implications for designing the best extraction condi-ions and minimizing the amount of co-solvent (ethanol) requiredn extracting the target flavonoid.

The use of compressed gases at conditions far above their crit-cal temperature as diluents in a highly compressed supercriticaluid such as SC-CO2 has been studied by several groups [36–38].uch binary mixtures of compressed gases may act as co-solventsr antisolvents depending on their Tc’s relative to that of supercrit-

cal fluid [39]. Helium and nitrogen have been previously studieds anti-solvents in supercritical fluid extraction of caffeine [40] andipids from soybean oil [41]. For the extraction of caffeine whensing supercritical carbon dioxide as a solvent, increasing the con-

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cal Fluids 47 (2009) 598–610

entration of nitrogen led to a dramatic reduction in solubility ofaffeine in CO2. Similar trends were recorded when helium wasdded to SC-CO2 for the solubility of soybean oil in SC-CO2. This hasmportant implications for conducting reactions in SC-CO2 sincehe addition of hydrogen to SC-CO2 permits higher hydrogenationeaction rates to be attained [42], but at the cost of reduced soluteolubilities in the more highly compressed supercritical fluid com-onent. Dissolved gases in compressed dense fluids or liquids canave a profound effect on the extraction of solutes from substratesing such modified solvents. So-called “enhanced fluidity mod-

fiers [43] serve as a basis for increased extraction of oils frometroleum reservoirs, analytes absorbed in sample matrices, and

n the continuous extraction of vegetable oils from seed meals.Recently, compressed gases in their supercritical fluid state; par-

icularly SC-CO2, dissolved in liquids and subcritical liquids haveecome of interest as in situ catalysts or modifiers for reaction andxtraction unit processing. The use of SC-CO2 as a replacement foretallic catalysts in glycerolysis reactions was reported by Temelli

t al. [44] and a review of its use in synthetic organic reaction chem-stry has been published by Rayner et al. [45]. Dissolving SC-CO2nder pressure in pressurized water creates a versatile mediumith respect to acidic-based extraction chemistry and reactions due

o the inherent carbonic acid equilibrium that is pressure depen-ent as studied by Toews et al. [46]. We and others have found that

f sufficient CO2 under pressure is applied to aqueous solutions,hat pH’s between 2.0 and 2.5 can be achieved. The basis of thisnhanced dissolution can be seen from the literature data that weave plotted in Fig. 8 [47–50]. Intuitively, increasing the tempera-ure of water should decrease the amount of gas dissolved in waters borne out by the data taken at lower pressures and temperatureshown in Fig. 8, however application of greater pressure as appliedo SC-CO2 lowers the pH of the solution due to an increase in themount of dissolved gas in water (Fig. 8). Our research group andthers have recently exploiting this trend to assist in the conversionf various types of carbohydrate-laden biomass by converting theonstituent carbohydrate polymers to lower oligomers for even-ual conversion to biofuels. Similarly control of solution pH byissolution of SC-CO2 can also affect the equilibrium-based specieshat is extracted when using subcritical water as is the case fornthocyanins and similar flavonoid-based solutes [51]. This tech-ique offers definite advantages with respect to avoiding the usef mineral acids in extraction and reaction chemistry since the dis-olved SC-CO2 can be jettisoned to the atmosphere or recycled byreduction in pressure. Studies using supercritical carbon dioxides reaction solvent especially in catalytic hydrolysis as describedbove have also shown good product separation characteristics byncreasing the pressure from 20 bar to as high as 120 bar. Approx-mately 90% of the hexanes were successfully separated from theydrolytic mixture dissolved in supercritical carbon dioxide [52].

It was remarked previously (Table 2) that it should be possi-le to use one critical fluid at different temperatures and pressureso perform multiple unit operations. For SC-CO2, it is well docu-

ented that changes in the fluid density can be used as a basisor the supercritical fluid-based fractionation (SFF) of a numberf complex mixtures. This is also possible in the case of subcrit-cal water, but usually through the adjustment of temperature.s the authors have previously noted, hot compressed water isversatile medium in both its sub- and supercritical regions by

sing it over a range of reduced temperatures (Tr) and pressuresPr) depending on the unit processing result that is desired. High

chemes, such as supercritical water oxidation, while a lower rangef Tr’s and Pr’s are employed for selective molecular transforma-ions such as biomass conversion via specific reaction pathways.iomass conversion in hot compressed aqueous media embrace

J.W. King, K. Srinivas / J. of Supercritical Fluids 47 (2009) 598–610 603

ater a

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Fig. 8. Mole fraction solubility of CO2 in w

perational parameters (Tr = 0.65–1.05, Pr = 0.35–2.0) in both theub- and supercritical phases for water The use of subcritical wateror extractions depends on the physical properties of the dissolvedolutes and their tendency to degrade under the chosen extractiononditions. For example, Tr’s and Pr’s are typically in the range of.50–0.80 and 0.02–0.35, respectively, for the extraction of naturalroducts.

Fig. 9 shows a hypothetical multi-unit processing scheme basedntirely on subcritical water for the treatment of a potentialiomass substrate [7]. Here the target substrate is pretreated withressurized water to prepare it for eventual extraction or reac-ion using subcritical water. The pretreatment with pressurizedater can be used to swell the substrate or comminute it forore effective extraction or reaction. Using the above criterion,

n extraction can be performed using pressurized water abovets boiling point to recover high value botanical extracts from theubstrate prior to using subcritical water as a reaction solvent. Post-xtraction treatment is then performed over a higher temperature

ange (150–300 ◦C) to hydrolyze the remaining biomass for furtheronversion to a lower molecular weight hydrolyzate suitable forermentation to a liquid fuel. Partial realization of this scheme wille described further in Section 4. The advantage of the described

Fig. 9. Tandem subcritical water processing of biomass.

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rocess is that it can be potentially conducted in one reactor ofntegrated processing plant therefore saving on capitalization costs.

. Multiple unit processing: concepts and possibilities

The generic concept of using critical fluids in more than onenit process utilizing critical fluids was discussed in Section 1 andpertinent example given. In this section we wish to elaborate on

his concept in two ways: (1) coupling critical fluids with an exist-ng processing device and (2) coupling unit processes to achieve

final product or end goal. The senior author has noted previ-usly that processing with supercritical fluids can consist of anxtraction mode (SFE), fractionating using supercritical fluids (SFF),onducting a reaction(s) in critical fluid media (SFR or CFR) and theormation of fine particles in critical fluid media, or CPT. Advances inrocessing using critical fluids can frequently be achieved by mod-

fying an existing device or concept, such as an expeller or extrudero provide a continuous processing method, or utilize equipmento concentrate or fractionate an initial extract or reaction mixture.

Here we shall consider two significant advancements utilizingompressed critical fluids with two devices: an expeller with a focusn seed oil processing and the use of membranes coupled withritical fluids.

The concept of continuous processing of oils from seeds andeals goes back to the mid-1980s with the description of the

peration of an Auger-type screw press by Eggers [53]. Here a super-ritical fluid such as SC-CO2 is used to assist in the removal of oilrom crushed seeds or meals which may have been partially pre-xtracted. The physicochemical basis of the process is still not wellnderstood, but involves the addition of liquefied CO2 to the seedsr meal inside an expeller barrel to aid in the oil extraction process.he hydraulic compression on the seed meal creates considerableressure and heat on seed matrix resulting in the conversion ofhe added CO2 to its supercritical state. The hot compressed carbon

ioxide partially solvates the seed oil akin to what occurs whenFE is performed with SC-CO2, but more importantly dilutes orxpands the expressed oil enhancing its removal from the seedr meal bed. Methods to achieve this goal have been describedn the patent literature most notably by Foidl [54] whose process

6 ercritical Fluids 47 (2009) 598–610

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04 J.W. King, K. Srinivas / J. of Sup

as been partially commercialized and applied to the processing ofoybeans. This CO2-assisted expression process has been commer-ialized by Crown Iron Works in Minneapolis, Minnesota under therademark of HIPLEX process and CO2 expression demonstrated onHarburg Freudenberger expeller having a 25 ton per day capac-

ty. The process is in commercial operation at SafeSoy Technologiesn Ellsworth, Iowa. The ratio of oil to CO2 is 3:1 which reduces theegetable oil viscosity by 1/10 resulting in between 80 and 90% veg-table oil recovery for soybeans and over 90% recovery of canolail. Such solvent-free oils and meals are superior in quality to sol-ent extracted products. A similar device would be welcomed forubcritical water extraction of natural and food-related productss well as for the conversion of biomass substrates on a contin-ous basis (see Section 4) in which the substrate to be extractedr treated with pressurized water would be contacting as a slurryith the pressurized water. This could be in principle applied to

uch diverse matrices as grape pomace, cocoa beans, and herbalubstances provided the residence times above the boiling point ofater are minimized. Current systems for affecting such pressur-

zed water extractions are staged as semi-continuous batch systemsr by combining the substrate to be processed as aqueous slurryith water before passage through a heated extraction vessel. It

hould be noted that critical fluid-based expeller processes com-ete with similar unit processing done with the aid of extruders55]. Although extruders have shown promise in the processingf finished food products [56] their attendant expense and lowerhroughputs make them less attractive than the expeller-based pro-esses described above.

The second development which we feel is important is the cou-ling of critical fluids with membrane technologies. This can bepplied in a number of very diverse applications ranging from con-entrating extracts, providing additional fractionation on extractserived from SFE using SC-CO2, and as an alternative post-SFE sep-ration process to avoid the recompression costs associated withhase-based separation methods involving recycle of the supercrit-

cal fluid after extraction. In the latter case, Brunner [57] has notedhat since the molecular weight of supercritical fluid is considerablyess than the extracted solutes, application of a permselective mem-rane that is compatible with operation at high pressure should beeasible. Coupling of supercritical fluid extraction processes with

embrane technology for supercritical fluid recovery and prod-ct purification can decrease the energy requirements and provideractionation of the extracts or reactants from an extraction and/oreaction processes. Sarrade et al. [58] has provided a comprehensiveeview on the integration of these two technologies.

Studies have also been conducted to study the performance ofifferent types of reverse osmosis (RO) membranes in recovery ofupercritical carbon dioxide after the extraction of essential oilsrom natural products like lemongrass, orange and nutmegs. Essen-ial oil retention by the membranes was reduced with increasen the trans-membrane pressure drop but there was no effectbserved when the oil feed concentration was varied. Recovery ofhe essential oils was approximately 90% via this coupled process59]. RO membranes were also used for the recovery of carbon diox-de from limonene extracted from essential oil yielding matrices.he process compared the performance of one nanofiltration andhree RO membranes at identical upstream and trans-membraneressures slightly above the Tc of CO2.

Integrating critical fluid technology with membranes hasermitted the separation of low and high molecular weight

ompounds obtained from the SC-CO2 extraction of lipids fromoodstuffs such as butter or fish oil using nanofiltration mem-ranes [60]. Similarly, it has been reported that is possible to extractolyphenols from cocoa seeds using neat SC-CO2 and with ethanols a co-solvent, and then concentrate the extract using polymeric

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ig. 10. Phospholipid enrichment/fractionation using coupled supercritical fluid-ased processes.

anofiltration or reverse osmosis membranes. This system oper-ted at a pilot scale between 8 and 15 MPa and at 40 ◦C resultedn a maximum yield of polyphenols of 43% when the pressureas optimized at 8 MPa using ethanol. The study also indicated aigh performance of all the membranes when the trans-membraneressure was maintained in excess of 1 MPa [61]. The ability tooncentrate extracted polyphenols using SC-CO2 extraction pairedith membranes suggest that a similar tandem involving sub-

ritical water–membrane coupling would be advantageous sincextraction with subcritical water results in a diluted extract. Thisoncept was first advanced by King [62] and noted in a US patentssued to Wai and Lang [63]. They suggested that SFE could bemplemented on a natural product matrix followed by subcriti-al water extraction sequentially on the same matrix and thenollowed by a membrane separator to yield a concentrate of thequeous extract. The authors view the actual implementation ofuch tandem technologies as critical to the further development ofhe application of subcritical water extraction for processing food,utraceutical, and natural products.

Countercurrent contacting of a liquid or SC-CO2 stream on one-ide of a permselective hollow fiber membrane to enrich aromaomponents from aqueous solutions may be a way of approach-ng the above problem. Sims has demonstrated such a processnd patented it as the POROCRIT process, and also applied it tohe sterilization of citrus juices among other applications [64].esearch involving the use of compressed solvents such as carbonioxide to extract fermentation products like ethanol and ace-one within a hollow fiber membrane reactor employ a similarpproach. In this study, a membrane fiber made of microporousydrophobic polypropylene was used in a single fiber membraneontactor and operating conditions maintained at 0.02–0.07 MParans-membrane pressure at ambient temperature. For aqueousinary feed solutions containing 10:10 wt% ethanol: acetone or:5 wt% ethanol: acetone solution. Recovery of ethanol rangedetween 5 and 14% while that for acetone ranged between 68 and6% depending on the CO2 flow rate using molar solvent to feedatios between 3 and 10 [65].

Combing various unit processing options using critical fluids tochieve an end goal involves looking at a matrix of possibilities andchieving optimization for all of the combined processes. In ourpproach over the past two decades, we and others have investi-ated and optimized the individual unit processes with an overallnal objective of producing a final result/product—it is less frequent

o find the discrete steps already combined, but examples of bothill be given in this section and Section 4. Several reviews have beenublished by the senior author regarding coupled processing [18],

ts application for nutraceutical [5] and lipid processing [66], and

J.W. King, K. Srinivas / J. of Supercritical Fluids 47 (2009) 598–610 605

Table 3Percentage amounts of PLs in extracts derived from SFE and SFC processing stagesa

Phospholipid type SFE stage SFC stage

Phosphatidylethanolamine 16.1 74.9Phosphatidylinositol 9.2 20.8PP

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hosphatidic acid 2.8 55.8hosphatidylcholine 15.6 76.8

a Relative to other eluting constituents (oil and unidentified peaks).

he use of reaction chemistry for conversion of agricultural materi-ls [1]. Today, production plants and toll refiners in addition to SFEffer one or more other unit processes for the refining of materials,lthough there is room for more expansion using multiple unit pro-essing. Perrut [67] has recently reported a pilot plant designed forulti-fluid extraction and reactions using either SC-CO2 or subcriti-

al water. Similarly, we have been able to alter bench scale extractornits using booster-type pumps, originally configured for SC-CO2,o allow extractions and reactions to be conducted with subcriticalater [68].

Laboratory investigations of potential multiple unit processingith critical fluids is frequently done by investigating one discreterocess followed by another, and realizing the potential for arrang-

ng these in a tandem manner to achieve the desired end product.or example, based on several studies focused on the enrichmentf a lecithin-phospholipid (PL)-containing fractions from seed oilxtracts [69–72], we have postulated an entire process based on SC-O2 with and without co-solvent addition employing SFE followedy displacement supercritical fluid chromatography (SFC) [73] ashown in Fig. 10. Here soybean flakes are initially extracted withC-CO2 to remove the oil from flakes, followed by extraction of theLs from the de-oiled flakes with a SC-CO2/ethanol mixture. Theecond extraction step produces an extract enriched in PLs since PLsre not appreciably soluble in neat SC-CO2, but can be selectivelyemoved from the flake matrix with the aid of ethanol as a co-olvent. As shown in Table 3, the second SFE using SC-CO2/ethanolroduces an extract containing a total 43.7% by weight of PLs. This

s a considerable enrichment relative to the concentration of theLs in the starting oil or seed matrix. Further PL enrichment isacilitated as noted above, by transferring this extract enriched inLs to an alumina preparative SFC column, where SC-CO2 modifiedith a 5–30 vol.%, 9:1/ethanol: water eluent is used to elute and

ractionate the PLs. In the case of the SFC enrichment step, eluentractions can be collected as a function of time and their PL contentuantitated. As indicated by the data given in Table 3, collectionf discrete fractions during the SFC process can produce purities inxcess of 75% for the individual PLs, phosphatidylcholine and phos-hatidylethanolamine. It should be noted in the described processhat in the SFC steps, only GRAS solvents are being used for thenrichment process. Recently, a similar SFE/SFC process has beenuggested for isolating sterols and phytosterol esters from corn brannd fiber [74].

To justify the above process we have quantified the degree of PLnrichment in terms of the product selling price after each unit pro-ess. Therefore starting from crude soybean oil at US$ 25–35/lb wean obtain a PL-lecithin-enriched fraction after SFE using ethanols a co-solvent worth US$ 31.00/lb, and subsequently with SFCroduce pure phospholipid extracts that retail for US$ 1800/lb.rogress toward the above goal has been partially realized by theonstruction of commercial plants to produce deoiled-lecithin byhde and Thar Technologies.

The above process is a SFE–SFC binary unit processing scheme.e have on a lab scale also demonstrated the feasibility of a

FR–SFR sequential set of reactions to make fatty alcohol mixtures.he generation of fatty acid methyl esters (FAMES) in this case

hptp

ig. 11. Production of fatty alcohol mixtures using a (a) SFR–SFR sequential reactioncheme, (b) feedback of methanol into the SFR process and its inherent “greenness”.

as based on studies involving the enzymatic synthesis of FAMESirectly from vegetable oils dissolved in SC-CO2 [75,76]. Combin-

ng this transesterification reaction with a hydrogenation reactionsing consecutively coupled packed bed reactors allowed the pro-uction of FAMES in either a SC-CO2 or SC-C3H8 stream followed byxhaustive hydrogenation of the FAMES to fatty alcohols as shownn Fig. 11a. In this process, a non-Cr catalyst was used to success-ully convert the FAMES to the C16 + C18 saturated alcohols at 250 ◦Cnd 25 mol% H2 in SC-CO2. This is an excellent example of how awo-step synthesis process can be conducted in supercritical fluid

edia that is environmentally benign by permitting the reuse ofhe critical fluid media as well as the reaction by-product from theydrogenation step, methanol (Fig. 11b).

It is impossible to separate in the multiple unit and fluid pro-essing platform the role of fluid interchange with tandem unitrocessing. Toward this end, the choice of solute or substrate modi-cation and/or fluid medium can enhance the opportunity to utilizehe various combinations of fluids or unit processes. Two will beited here: (1) formation of methyl esters of lipid-type solutes suchs fatty acids (FAMES), and (2) use of water primary for hydrolysisf complex naturally occurring substrates. FAMES are an extremelyersatile modification for the critical fluid processing of fats/oilsnd their oleo chemical derivatives. Aside from the formation ofAMES for conversion to biodiesel via enzymatic synthesis [77] or inub- and supercritical methanol [78], FAMES or similar esters can besed advantageously in SFE [79], columnar modes of SFF [80], SFC81], and as noted above in SFR. This versatility is due to one or moref the following factors relative to the non-methylated analogs: (1)nhancement of solute volatility or solubility, (2) improvement ofeparation factor (˛), (3) intermediate formation for downstreamynthesis, and analytically useful derivatives [15]. Formation ofAMES before utilizing multi-unit processing can allow easier SFFf fatty acids [82], selective SFE and SFR of fatty acids from tallil [83] for subsequent conversion to biodiesel, and to fractionateoapstsock [84] or deodorizer distillate [85]. For example, coun-ercurrent multistage processing of edible oils using critical fluids

as been shown to be capable of producing fatty acid esters, toco-herols, squalene, sterols, and triglycerides. Approximately 70% ofhe fatty acid methyl esters in deodorizer distillates plus toco-herols and sterols can be extracted with SC-CO2 [86]. Tocopherols

606 J.W. King, K. Srinivas / J. of Supercriti

Fa

atS

abHwLbrSbaahfrt

4

iPSmS[sopdsAdsm

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cesSparcalfibatpsTcapmisiisafsca

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ig. 12. Treatment of various types of biomass and mixtures with subcritical waternd the resultant products.

nd sterols in the resultant extract can then be separated fromhe FAMES by columnar SFF by countercurrent fractionation usingC-CO2.

Illustrated in Fig. 12 are a number of potential sequences forpplying subcritical water to pre-fractionated and unfractionatediomass substrates for the production of fuels and chemicals.ydrolysis as illustrated can produce oils or lipids by subcriticalater extraction followed by SFR to produce free fatty acids (FFA).

ikewise, subcritical water on isolated protein fractions (perhapsy subcritical water extraction) can be used as a time-dependenteaction medium to produce protein hydrolyzates, an extendedFR to produce peptide mixtures, etc. If one takes unfractionatediomass mixtures, such as a deoiled-protein/carbohydrate fraction,nd treats it using SFR with subcritical water, mixtures of aminocids, organic acids, and sugars results. While the resultant mixedydrolyzate might not seem useful it is in actuality a seed mixtureor the eventual fermentation production of biomethane, since theesultant hydrolyzate can be treated with methanogenic bacteriao produce a mixture of CO2 and CH4 [87].

. Examples of integrated critical fluid processing

There are numerous examples of multi-fluid and -unit process-ng but they can be crudely classified into several generic groups.rocessing using one critical fluid offers the opportunity to mix SFE,FF and SFR processes as has already been amply illustrated. Theost versatile combination seems to be using two fluids, namely

C-CO2 and pressurized water. Using subcritical water, Smith et al.88] have shown that the “green” processing of cashew nuts is pos-ible; SC-CO2 being used for the recovery of the cashew nut shellil containing the pharmacological-active alkenyl phenolic com-ounds. The cashew nut shells were initially treated with carbonioxide at 40 MPa and 100 ◦C using successive depressurizationteps to accelerate to remove 95% of the cashew nut shell liquid.fter SC-CO2 extraction, the residual shell biomass can be rapidlyissolved (5 min) in pressurized high temperature water (410 ◦C)uggesting that hydrolytic-based gasification of remaining shell

aterial would be suitable for fuel purposes.Numerous research groups are now practicing tandem SC-

O2–subcritical water extraction using different approaches.Hybrid” SFE with CO2 and water has been applied to a wide varietyf natural product matrices [89] such as the separation of pesti-

p

cit

cal Fluids 47 (2009) 598–610

ides from ginseng using an upward flow of CO2 while ginsengxtracts are concentrated in a downward flow of water using theame SFE vessel. Wai and coworkers [90] have sequentially appliedC-CO2 and then subcritical water for fractionating bioactive com-onents from St. John’s wort. The active components hyperforinnd adhyperforin were partitioned into CO2 at 30 ◦C and 8 MPaesulting in 93–99% recoveries, while hypercin and pseudohyper-in were subsequently recovered via pressurized water extractiont neutral pH at 80–93% yield. Similarly, the potential and prob-ems of sequential extraction of the South American plant, boldo,rst using SC-CO2 with and without ethanolic co-solvent, and thenatch-pressurized water extraction were reported by del Valle etl. [91]. The extraction selectivity for the plant essential oil andhe boldine alkaloid paralleled their relative solubility in non-olar SC-CO2 and enhancement in the solubility of more polarolutes in CO2 + ethanol mixtures as well as in subcritical water.his study and others involving polyphenolic target solutes, indi-ate that extractions with subcritical liquids are effective up tocertain extraction temperature, but beyond this temperature,

ressurized solvent processing can lead to solute degradation orolecular transformation [92–94]. In the latter cases, this reactiv-

ty of the solute in the subcritical liquid maybe viewed as desirableince the free radical scavenging capacity of the resultant extractsncrease. An example of this trend was reported by the Ibanez groupn Spain [95,96] for the extraction of rosemary by either SC-CO2 orubcritical water. Carbon dioxide fractions collected at 10–40 MPand 40–60 ◦C resulted in removal of the essential oil and partialractionation of the antioxidants from the rosemary matrix, whileubcritical water favored extraction of the more polar bioactiveompounds such as carnosic acid or resultant fractions having highntioxidant capacity.

The above typical results of employing CO2 extraction in tandemith pressurized liquid fluids suggest an interesting option and cur-

ent trend in employing this mixed pressurized fluid matrix as bothxtraction and reaction media. As noted previously, the incorpora-ion of pressurized CO2 into subcritical water, i.e., a gas-expandediquid, makes for an interesting extraction and reaction medium.he Meireles group in Brazil [97,98] have utilized this principle inhe processing of ginger bagasse both as a pre-treatment step ando degrade bagasse to sugars for potential fermentation. Pretreat-

ent with SC-CO2 seemed to yield somewhat ambiguous resultsince the authors state that non-treated bagasse was hydrolyzedore effectively then CO2-pretreated bagasse [99]; the latter pro-

ess was hypothesized to degrade the oleoresinous materials inhe bagasse matrix. Their results for matrix pretreatment with SC-O2 are in stark contrast with other reports in the literature that

ndicate SC-CO2 pretreatment is an effective procedure preced-ng biomass degradation [100]. It should be noted that the aboveesults are somewhat different then using the previously men-ioned carbonated water to hydrolyze carbohydrate polymers; alsoC-CO2 is effective in removing high value components from theiomass matrix prior to hydrolyzing the biomass matrix [99]. Sinceeported SC-CO2 pretreatment methods exist and carbonated waterydrolysis has been shown by us [101] and others [102] to be anffective hydrolysis medium, the above ambiguity may be due tohe varying recalcitrance of the target biomass matrix to hydrolyticegradation. The hydrolytic action patterns of carbonated water canary quite significantly depending on the matrix being hydrolyzedlthough the hydrolysis temperature and residence time can bearied to produce optimal depolymerization of the carbohydrate

olymers inherent in the biomass.

One aspect of our current research focuses on the application ofritical fluids for processing grapes and grape by-products and sim-lar natural antioxidant-containing matrices. These matrices andarget solutes are a fruitful area in which to apply combinations

ercritical Fluids 47 (2009) 598–610 607

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J.W. King, K. Srinivas / J. of Sup

f mixed critical fluid and unit processing steps. One of the sem-nal questions is whether SC-CO2 and co-solvent combinations orhot pressurized fluid such as water or ethanol – or combinations

hereof – are most appropriate for extracting and fractionating theargeted solutes. There is a considerable literature in the appli-ation of SC-CO2 for extracting grape seed oil [103,104] as wells further fractionating the extract to enrich certain polypheno-ic constituents. Recovery of solutes such as gallic acid, catechin,picatechin, etc. via a SC-CO2-based method almost always requirehe use of methanol or ethanol as co-solvents [105].

Other studies have utilized subcritical water to extract procyani-in compounds and catechins from grape processing wastes [106].xtractions conducted at approximately 10 MPa and in the temper-ture range of 50–150 ◦C were adequate to recover and fractionateallic acid, procyanidin dimers, and the corresponding oligomersrom the grape pomace using an analytical scale pressurized fluidxtractor (ASE). Aside from water and hydroethanolic pressurizeduid extraction, sulfured water has also been proven effective

or the extraction of anthocyanins and procyanidins from grapeomace [94]. This parallels similar work by the senior author insing neat and acidified water to extract anthocyanins from berryubstrates using both ASE and a batch continuous subcritical waterxtractor. As noted in the patent issuance on this process [107], res-dence time of the extracted solute in the hot pressurized water

ust be minimized to prevent degradation of the anthocyaninoieties or their possible reaction with sugars to other products.

t is unknown at this time whether such side reactions in pres-urized water could be generating antioxidant moieties, but theotential ability to control the ratio of polyphenolic stereoisomersnd to depolymerize or repolymerize biologically active antioxi-ant oligomers in pressurized fluid media could be a significantrea for future research—particularly if they come from cheap andenewable natural resources [8].

In concluding this section it is worth considering far moreanging applications of the critical fluid technology platform thatmbrace the multi-fluid and -unit processing as discussed aboveor the industrial production of chemicals, fuels, etc. The authorsave previously noted the considerable potential for applying crit-

cal fluids in biorefineries, etc. [7] but several existing examples areorth citing. Kusdiana and Saka [108] have demonstrated a two-

tep process for the production of biodiesel (see Fig. 13b) basedn the Saka—supercritical methanol process (Fig. 13a) for convert-ng both fats/oils and free fatty acids to biodiesel. The Saka-Dadanrocess uses subcritical water in front of the Saka process for theydrolysis of fats/oils to free fatty acids followed by a more benignupercritical methanolysis of the resultant free fatty acids to FAMES.pilot scale unit of this process is in operation in Fuji City, Japan.

his overall biodiesel production platform is an excellent examplef having a critical fluid-based SFR–SFR integrated process.

More recently Brunner and colleagues [109,110] have demon-trated an integrated critical fluid-based process for the productionf several chemicals from rice bran. Although studied in discreteteps, the foresight of this effort towards developing an integratedritical fluid processing scheme is impressive. Rice bran is ini-ially deoiled using SC-CO2 at the appropriate extraction conditionsesulting in defatted rice bran. This substrate is then subjected toubcritical water hydrolysis at approximately 250 ◦C to partiallyretreat and depolymerize the rice carbohydrate lignocellulosicolymers to lower oligomers. Final conversion of the subcriticalater hydrolyzate to glucose and xylose is accomplished with a

ixed enzyme cocktail for subsequent fermentation conversion to

ioethanol. The resultant 5–10% aqueous ethanol mixture from fer-entation is then multi-staged contacted counter currently with

C-CO2 to yield 99.8% pure ethanol. It is envisioned that non-ermentable residues left over from the hydrolysis process could

oto

ig. 13. The Saka-Danan two-step SFR process incorporating both subcritical waterydrolysis and supercritical methanolysis of fats/oils to produce biodiesel.

e further treated with the aid of sub- and supercritical fluids toroduce biogas.

Baig et al. [111] have recently reported on the combined criticaluid treatment of sunflower oil to yield value-added substances asell as a model for the “critical fluid biorefinery”. This concept cane achieved by coupling two or more reaction processes into oneontinuous flow system; namely the subcritical water hydrolysisf sunflower oil triglycerides to free fatty acids followed by ester-fication of the free fatty acids to FAMES in SC-CO2 using lipaseatalysis. The subcritical water extractor conditions were main-ained at 250–390 ◦C with pressures as high as 10–20 MPa usingil:water ratios of 50:50 and 80:20 (v/v). The supercritical fluidnzymatic-based esterification process was operated at tempera-ures 40–60 ◦C using a Novozyme enzyme catalyst. The subcriticalater studies indicated a high rate of conversion at higher temper-

tures (330 ◦C) followed by possible degradation of the free fattycids when exposed to longer residence times. A yield of approx-mately 90% hydrolyzed free fatty acids was achieved in 25 min at30 ◦C or for 45 min at 310 ◦C. The esterification process yieldedetween 60 and 70% FAMES at a pressure of 20 MPa and 60 ◦C with

ow enzyme concentrations.

. Concluding remarks

In this document we have attempted to offer our perspectivef the current state of integrated critical fluid processing. It ishe author’s opinion that a positive future exists for combiningur existing knowledge and studies involving discrete fluids and

608 J.W. King, K. Srinivas / J. of Supercritical Fluids 47 (2009) 598–610

ed wa

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Fig. 14. Process flow diagram for integrated carbonat

nit processes done under critical fluid conditions. This will partlyequire “thinking outside the box” in terms of how industrial highressure processing facilities are constructed and by realizing thathe costs inherent in the core pumping, liquefaction, and compres-ion facility can serve more than just one purpose, i.e. SFE, and cane applied with more than just one critical fluid medium.

Over the past twenty years the senior author has on more thanne occasion been queried on the possibility of constructing a SFElant in close proximity to an alcoholic fermentation facility thatroduces high purity CO2 as a by-product. This would seem logi-al since the opportunity to apply SFE for vegetable or specialty oilxtraction could be facilitated with this source of CO2 as well as anyf the above mentioned CO2-based unit processes. The productionlso of ethanol at such a site facilitates a preferred co-solvent foroupling with CO2 as documented previously. Today in the renew-ble bioenergy field it is envisioned to build coexisting bioethanolnd biodiesel production capabilities at the same site. Hence basedn our discussion above, this suggests the possibility of extendinghe application of critical fluids platform for the production of thesewo renewable fuels as documented above. Similarly, it was notedbove that SC-CO2 could be mixed advantageously with pressurizedater for extraction and reaction chemistry.

Fig. 14 shows a hypothetical process flow diagram for anntegrated carbonated water pretreatment process for bioethanolroduction. Here the process flow diagram illustrates not onlyhe sequence of operations associated with a modified carbon-ted water pretreatment–bioethanol production plant, but alsohe elimination of several processing steps required in current

ioethanol production practice. Also illustrated are the feedback

oops from the CO2 production from the fermentor as well asecompression of CO2 after the carbonated pretreatment stage.ote that the CO2 can be recovered and fed back to the CO2 storageessels, then recombined with water and pressurized to provide

ter pretreatment process for bioethanol production.

high pressure aqueous medium for pretreatment and hydroly-is. The pictured process flow diagram is similar to that reportedor current bioethanol production however the addition of acid forydrolysis has been eliminated as well as the corresponding neu-ralization agent and the attendant use of gypsum as a neutralizinggent. Note that in Fig. 14, lignin is separated and used as boiler fuel.owever it should be possible to treat lignin at higher pressuresnd temperatures with sub- or supercritical fluid water reactiono also produce valuable carbochemicals [7]. Such a critical fluid-ased processing concept supports the use of renewable resources,sustainability platform, and does so in a “green” environmentallyenign manner.

cknowledgments

We should like to thank Dr. Charles Hansen of Horsholm, Den-ark for guidance in the use of the extended solubility parameter

oncepts presented in the cited associated software programs. Mr.homas Potts is acknowledged for preparing some of the graphicshat appear in this manuscript. Our collaboration with Professoruke Howard of the Department of Food Science, University ofrkansas is also gratefully acknowledged.

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